1. Field of the Invention
The present invention relates to a receiver that receives a radio signal including N possible symbols (where N is an integer having a value of at least two) each represented by M chips (where M is an integer having a value of at least two), and detects the degree of correlation with a symbol serving as a reference, and its receiving method.
2. Related Background Art
In the CCK (Complementary Code Keying) modulation scheme adopted in IEEE802.11b, signal modulation is conducted by using 64 possible symbols each represented by 8 chips.
An example in which a plurality of correlation processing units for detecting the degree of correlation of the symbols included in the radio signal and a symbol determination unit for determining the symbols are provided is disclosed in Japanese Patent Laid-open 217789/2002.
The symbol determination unit selects the maximum output among the outputs of a plurality of correlation processing units, and detects a phase of the output of the selected correlation processing unit. Thereby, {φ1, φ2, φ3, φ4} dependent on data bits is decided, and the symbols are determined.
In a multipath environment in which a delay wave exists, however, cross-correlation characteristics between symbols deteriorates by the influence of the delay wave, and the output of the correlation processing unit corresponding to a transmission symbol does not become the maximum.
For example, it is supposed that both a preceding wave w1 of a certain symbol and a one-chip delay wave of the preceding wave w1 are received by the receiver. In this case, originally, only the output of the correlation processing unit having a symbol corresponding to the preceding wave w1 should become the maximum. In a multipath environment having the preceding wave w1 and a one-chip delay wave w2 as a mixture, however, both the output of the correlation processing unit having the symbol corresponding to the preceding wave w1 and an output of a correlation processing unit having a symbol corresponding to the one-chip delay wave become large. Therefore, there is a likelihood that the transmission symbol cannot be discriminated uniquely by using only outputs of the correlation processing units.